The state of the art comprises a wide variety of treatment methods with the objective of performing refractive correction of the human eye. The objective of the operation methods is to systematically reshape the cornea in order to influence the light refraction in the eye. Various operation methods are used for this purpose. The most widespread implementation is the so-called Laser-Assisted In Situ Keratomileusis, also abbreviated LASIK. In this procedure a flap is cut from the surface of the cornea on one side and folded back. This flap can be cut by means of a mechanical microkeratome or else by means of a so-called femtosecond laser keratome of the type sold, for example, by Intralase Corp. of Irvine, Calif., United States. Once the flap has been cut and folded back, the LASIK surgery provides for the use of an excimer laser that ablates the cornea tissue underneath the flap that has been exposed in this manner. After the volume located underneath the surface of the cornea has been vaporized in this manner, the cornea flap is folded back into its original position.
The use of a laser keratome to expose the flap is advantageous in comparison to employing a mechanical blade since the geometrical precision is improved and the frequency of clinically relevant complications is diminished. In particular, the flap can be created with a much more constant thickness by using laser radiation. Moreover, the edge of the incision is precisely shaped, reducing the risk of healing problems due to this boundary surface that is left over after the operation. However, a drawback of this method is that two different treatment devices have to be used, namely, the laser keratome that exposes the flap and the laser that vaporizes the cornea tissue.
These drawbacks are overcome with a method that was recently implemented by Carl Zeiss Meditec AG and that is abbreviated as FLEx (Femtosecond Lenticule Extraction). This method for lenticule extraction employs a short-pulse laser, preferably a femtosecond laser, to create an incision geometry in the cornea, separating a cornea volume (so-called lenticule) in the cornea. This volume is then removed manually by the surgeon after the flap covering the lenticule has been folded back. The advantage of this method is, for one thing, that the quality of the incision is further improved by using a femtosecond laser combined with a curved contact glass. Secondly, only a single treatment device is needed; the excimer laser is no longer used. This method also avoids risks and limitations of the excimer laser.
A refinement of the FLEx method is referred to in the literature today as the SMILE (Small Incision Lenticule Extraction) method in which no flap is created but rather only a small opening incision that serves to access the lenticule located beneath the so-called cap. The separated lenticule is removed through this small opening incision, as a result of which the biomechanical integrity of the anterior cornea is less affected than in the LASIK, or similar methods. Moreover, in this manner, fewer superficial nerve fibers in the cornea are cut and this is likely to be advantageous when it comes to the restoration of the original sensitivity of the surface of the cornea. As a result, the symptom of dry eyes that often has to be treated after a LASIK procedure is often less severe and less protracted. Other complications after LASIK, which usually have to do with the flap (e.g. displaced flap, folds, epithelial ingrowth in the flap bed) occur less often in procedures without a flap.
When incision surfaces are made in the cornea by means of laser radiation, the optical radiation effect is normally utilized in that a photodisruption is created by means of individual optical pulses whose duration can be between about 100 fs and 100 ns. It is also a known procedure to apply individual pulses whose energy lies below a threshold value for a photodisruption into the tissue or material with such an overlap that this also achieves a material or tissue separation. This concept for creating an incision in the cornea tissue allows a wide variety of incisions. For clinical reasons, it can be advantageous for the diameter of the cap incision, which defines the anterior surface of the lenticule to be selected larger than that of the lenticule incision, which defines the diameter and posterior surface of the lenticule. See, for example, US Patent Application Publication No. 2014/0128855.